Technical Abstract:
The etiology of respiratory disease is complex. Multifactorial in origin, respiratory disease results from the interaction of stress, multiple viruses, and multiple bacteria. A wide variety of different stressors and agents can be involved in the disease process. Healthy ruminants can carry one or more of the viral and bacterial agents in their upper respiratory system with no apparent ill effects. Some of the agents produce only mild clinical signs by themselves, but in combination they may cause severe signs and death. Among domestic ruminants it is typically the cumulative effect of several physical, psychological, and infectious stressors that overcome the animal’s innate and adaptive immunity and thereby establish respiratory disease. Viruses affecting the respiratory tract of ruminants are widespread and numerous. Although acute respiratory infections in bighorn sheep have not been well studied, a number of viruses have been identified either through virus isolation or by serology: parainfluenza type 3 virus; respiratory syncytial virus; bovine herpes virus type 1; bovine viral diarrhea virus; bluetongue virus/epizootic hemorrhagic disease virus; and contagious ecthyma virus. Although viral agents are recognized as primary pathogens in ruminant respiratory tract disease, they often strongly predispose to secondary bacterial infections by facilitation of colonization, adherence, and invasion. Viral damage to local respiratory tract epithelium impairs local defense mechanisms, and couples with systemic stress and immunosuppression to further predispose the animals to opportunistic bacteria. Once established, the aggressive nature of respiratory bacteria infections (typically Mannheimia haemolytica, Pasteurella trehalosi, and Pasteurella multocida in bighorn sheep), makes it difficult to determine whether there is a primary underlying viral etiology.